I do not know if that is useful, but I will reply.
The easiest trouble with writing from TVPaint is the case where "space key" is included in the layer or instance or file name.
Please check once that there are elements that contain "Space key" in the timeline.

And,
if (ReadIntFromData (dataTree, "version.major", 0)! = scriptVersionMajor) ...
Line 2 and line 3 of the JSON file
"version": {
"major": 5,
Refer to. I think that it stops its operation at this line only if JSON does not have this element at all, so please check if it is included.

I appreciate Hironori Takagi 's reply above , but could someone from TVPaint development team reply to David Meslin's question ?
Is this a known issue and can you confirm that JSON Import JSX Script does not work in AE 2019 , because I am about to
update to AE 2019 , but I don't want to update if this is still a problem.

I added After Effects CC 2019 to my current environment (TVPaint 11.0.8, Windows 10 Pro, DELL Precision M 3800) and tried it, Import-json worked normally.
I think that it is preferable to submit samples of exported json and folders from users who do not work.

Bumping this up from David Meslin's unanswered post on Feb. 04, 2019 .

I have recently updated my After Effects to CC 2019 (v16.1.1 - Build 4) and I am sorry to report that the TVPaint JSON import script does not work in AE CC 2019 with Mac OS 10.12 Sierra.
I am using version 5.7 of the After Effects Import script.

Screen Shot 2019-06-18 at 3.57.46 PM.jpg (23.7 KiB) Viewed 1179 times

Could someone from the TVPaint Development Team please respond to confirm that this is a known issue and is there any fix in the works ?

I've run some tests on both Windows 10 (I don't think we have any Windows 7 left, we all made the swtich to W10) and macOS Sierra 10.12.6.
The After Effects version used for the test was AE 2019, v16.1.2, Build 55. I used the JSOn export in TVPaint 11.0.8 to export one of my tvpp files so the JSON file is a newly created one.

On Windows 10, no issue: the Import panel is there, I can browse and select a JSON file, import it, and have the TVPaint structure re-created in After Effects.
On macOS Sierra, it's the same, it works: